A new P-T-t path of eclogites from Chinese southwestern Tianshan: constraints from P-T pseudosections and Sm-Nd isochron dating
A new P-T-t path of eclogites from Chinese southwestern Tianshan: constraints from P-T pseudosections and Sm-Nd isochron dating
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中国西南天山榴辉岩的新P-T-t路径：P-T伪剖面和Sm-Nd等时线测年的约束
DOI：
10.1016/j.lithos.2014.04.009
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发表时间：
2014-07
期刊：
影响因子：
3.5
通讯作者：
Bader Thomas
中科院分区：
文献类型：
--
作者：
Du Jin-Xue;Zhang Li-Fei;Shen Xiao-Jie;Bader Thomas
西南天山造山带是地球上罕见的超高压（UHP）变质岩石之一，来自俯冲洋壳。为了解决其P-T演化的局限性和争议，本研究选择了三个代表性样品，两个钠长-绿帘石榴辉岩（H505-26和H76-10）和一个蓝闪石-多硅白云母榴辉岩（H711-1），Sm-Nd年代学和详细的相平衡模拟相结合。榴辉岩中石榴子石变斑晶具有保存完好的生长分带，向边缘的镁铝榴石含量增加，锰铝榴石含量减少。相平衡模拟结果表明，所研究的石榴石生长在硬柱石榴辉岩相，在榴辉岩H505-26在加热和埋藏，在榴辉岩H76-10和H711-1在加热和减压。三种榴辉岩的460-590 °C温度低于Sm-Nd系统的闭合温度，因此，石榴子石-绿辉石-全岩Sm-Nd等时线年龄具有反映石榴子石生长的潜力，榴辉岩相变质作用3个榴辉岩的Sm-Nd等时线年龄值一致，分别为309 ± 4.6Ma（H505-26）、306 ± 15 Ma（H76-10）和305 ± 11 Ma（H711-1），反映了高压榴辉岩相变质作用。根据新的年代学、详细的相平衡研究和已发表的资料，预测了中国西南天山榴辉岩的顺时针P-T-t轨迹，包括Pmax后加热-减压、随后的等温减压和最终的冷却-减压。南天山古洋俯冲作用开始于346 Ma以前，俯冲洋壳榴辉岩相变质作用发生在346-333 Ma，榴辉岩相变质作用高峰期在320-305 Ma，洋壳浮力折返作用发生在296-226 Ma。
The southwestern Tianshan orogenic belt hosts one of the Earth’s rare occurrences of ultrahigh-pressure (UHP) metamorphic rocks derived from subducted oceanic crust. To address its poorly constrained and controversialP-T-tevolution, for this study, three representative samples, two paragonite-epidote eclogites (H505-26 and H76-10) and one glaucophane-phengite eclogite (H711-1) are selected, and Sm-Nd geochronology and detailed phase equilibria modeling are combined. Garnet porphyroblasts from the three studied eclogites have well-preserved growth zoning with rim-ward increasing pyrope and decreasing spessartine contents. Phase equilibria modeling reveals that the investigated garnets grew in the lawsonite eclogite facies; in eclogite H505-26 during heating and burial, and in eclogites H76-10 and H711-1 during heating and decompression. Temperatures of 460–590 °C derived for the three eclogites are below the closure temperature of the Sm-Nd system, and, thus, the garnet-omphacite-whole rock Sm-Nd isochron ages have the potential to reflect garnet growth, i.e., eclogite facies metamorphism. The three eclogites yield consistent Sm-Nd isochron ages of 309 ± 4.6 Ma (H505-26), 306 ± 15 Ma (H76-10) and 305 ± 11 Ma (H711-1), reflecting HP eclogite facies metamorphism. Based on the new geochronology, detailed investigation of phase equilibria and published data, a clockwiseP-T-tpath with post-Pmaxheating – decompression, subsequent isothermal decompression and final cooling – decompression is predicted for the eclogites from Chinese southwestern Tianshan. We propose, subduction of the South Tianshan paleo-ocean began before 346 Ma, the eclogite facies prograde metamorphism of the subducting oceanic crust occurred at 346–333 Ma, peak eclogite facies metamorphism at 320–305 Ma, and buoyancy-driven exhumation of the oceanic crust at 296–226 Ma.
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影响因子：
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作者：
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通讯作者：
Lifei Zhang;Z. Lü;Guibin Zhang;Shuguang Song
影响因子：
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